Leucine zipper motif of chicken histone acetyltransferase-1 is essential for in vivo and in vitro interactions with the p48 subunit of chicken chromatin assembly factor-1

We cloned cDNA encoding chicken cytoplasmic histone acetyltransferase-1, chHAT-1, comprising 408 amino acids including a putative initiation Met. It exhibits 80.4% identity to the human homolog and possesses a typical leucine zipper motif. The glutathione S-transferase (GST) pull-down assay, involving truncated and missense mutants of the chicken chromatin assembly factor-1 (chCAF-1)p48, revealed not only that a region (comprising amino acids 376–405 of chCAF-1p48 and containing the seventh WD dipeptide motif) binds to chHAT-1 in vitro, but also that mutation of the motif has no influence on the in vitro interaction. The GST pull-down assay, involving truncated and missense chHAT-1 mutants, established that a region, comprising amino acids 380–408 of chHAT-1 and containing the leucine zipper motif, is required for its in vitro interaction with chCAF-1p48. In addition, mutation of each of four Leu residues in the leucine zipper motif prevents the in vitro interaction. The yeast two-hybrid assay revealed that all four Leu residues within the leucine zipper motif of chHAT-1 are necessary for its in vivo interaction with chCAF-1p48. These results indicate not only that the proper leucine zipper motif of chHAT-1 is essential for its interaction with chCAF-1p48, but also that the propeller structure of chCAF-1p48 expected to act as a platform for protein–protein interactions may not be necessary for this interaction of chHAT-1.     

Candidate Gene Identification with SNP Marker-Based Fine Mapping of Anthracnose Resistance Gene Co-4 in Common Bean

Anthracnose, caused by Colletotrichum lindemuthianum, is an important fungal disease of common bean (Phaseolus vulgaris). Alleles at the Co–4 locus confer resistance to a number of races of C. lindemuthianum. A population of 94 F_4:5 recombinant inbred lines of a cross between resistant black bean genotype B09197 and susceptible navy bean cultivar Nautica was used to identify markers associated with resistance in bean chromosome 8 (Pv08) where Co–4 is localized. Three SCAR markers with known linkage to Co–4 and a panel of single nucleotide markers were used for genotyping. A refined physical region on Pv08 with significant association with anthracnose resistance identified by markers was used in BLAST searches with the genomic sequence of common bean accession G19833. Thirty two unique annotated candidate genes were identified that spanned a physical region of 936.46 kb. A majority of the annotated genes identified had functional similarity to leucine rich repeats/receptor like kinase domains. Three annotated genes had similarity to 1, 3-β-glucanase domains. There were sequence similarities between some of the annotated genes found in the study and the genes associated with phosphoinositide-specific phosphilipases C associated with Co-x and the COK–4 loci found in previous studies. It is possible that the Co–4 locus is structured as a group of genes with functional domains dominated by protein tyrosine kinase along with leucine rich repeats/nucleotide binding site, phosphilipases C as well as β-glucanases.     

Conserved Linkage within a 4-Cm Region of Mouse Chromosome 9 and Human Chromosome 11

A six-point cross was carried out to determine the gene order and distances among loci on mouse chromosome 9. Our results are consistent with the following arrangement: centromere – Lap-1 – (1.2 ± 0.8) – Es-17 – (3.0 ± 1.0) – Ups – (1.3 ± 0.7) – Alp-1 – (23.1 ± 3.4) – Mod-1 – (10.9 ± 2.6) – Acy-1 . This study provides the first estimate of the distances between Es-17, Ups and Alp-1. Exceptions to the preferred association of alleles of Es-17 and Ups have been found in three feral populations and one inbred strain. Evidence is presented for the homology of this chromosome region with the ESA4 – UPS – APO-AI region on the long arm of human chromosome 11.     

Diversification of Paralogous α-Isopropylmalate Synthases by Modulation of Feedback Control and Hetero-Oligomerization in Saccharomyces cerevisiae

Production of α-isopropylmalate (α-IPM) is critical for leucine biosynthesis and for the global control of metabolism. The budding yeast Saccharomyces cerevisiae has two paralogous genes, LEU4 and LEU9, that encode α-IPM synthase (α-IPMS) isozymes. Little is known about the biochemical differences between these two α-IPMS isoenzymes. Here, we show that the Leu4 homodimer is a leucine-sensitive isoform, while the Leu9 homodimer is resistant to such feedback inhibition. The leu4Δ mutant, which expresses only the feedback-resistant Leu9 homodimer, grows slowly with either glucose or ethanol and accumulates elevated pools of leucine; this phenotype is alleviated by the addition of leucine. Transformation of the leu4Δ mutant with a centromeric plasmid carrying LEU4 restored the wild-type phenotype. Bimolecular fluorescent complementation analysis showed that Leu4-Leu9 heterodimeric isozymes are formed in vivo. Purification and kinetic analysis showed that the hetero-oligomeric isozyme has a distinct leucine sensitivity behavior. Determination of α-IPMS activity in ethanol-grown cultures showed that α-IPM biosynthesis and growth under these respiratory conditions depend on the feedback-sensitive Leu4 homodimer. We conclude that retention and further diversification of two yeast α-IPMSs have resulted in a specific regulatory system that controls the leucine–α-IPM biosynthetic pathway by selective feedback sensitivity of homomeric and heterodimeric isoforms.     

Three new species of the genus Aporcelaimoides Heyns, 1965 from Vietnam (Nematoda,Dorylaimida, Aporcelaimidae), with an updated taxonomy of the genus

Three new species of Aporcelaimoides from natural habitats in Vietnam are studied, described and illustrated, including line drawings, LM and/or SEM pictures. Aporcelaimoides brevistylum sp. n. is characterized by its body 1.95–2.90 mm long, lip region offset by deep constriction and 17–18 µm broad, ventral side of mural odontostyle 11–14 µm long with aperture occupying 62–71% of its length, neck 663–767 µm long, pharyngeal expansion occupying 58–66% of total neck length, uterus a simple tube 85–182 µm long, pars refringens vaginae absent, V = 55–63, tail short and rounded (34–46 µm, c = 49–76, c’ = 0.6–0.8), spicules 67–86 µm long, and one ventromedian supplement out the range of spicules. Aporcelaimoides minor sp. n. is distinguished in having body 2.09–2.61 mm long, lip region offset by deep constriction and 19–20 µm broad, mural odontostyle 14–16 µm long at its ventral side with aperture occupying 73–84% of its length, neck 579–649 µm long, pharyngeal expansion occupying 57–66% of total neck length, uterus a simple tube 44–69 µm long, pars refringens vaginae well developed, V = 48–56, female tail very short, rounded conoid or truncate (14–26 µm, c = 90–146, c’ = 0.3–0.6), and male unknown. Aporcelaimoides silvaticum sp. n. is characterized by its body 2.09–2.60 mm long, lip region offset by depression and 17–18 µm broad, mural odontostyle 11–12 µm long at its ventral side with aperture occupying 60–66% of its length, neck 597–720 µm long, pharyngeal expansion occupying 58–64% of total neck length, uterus a simple tube 128–243 µm long, pars refringens vaginae well developed, V = 58–60, tail short and rounded (27–37 µm, c = 67–94, c’ = 0.6–0.7), spicules 64–75 µm long, and two or three widely spaced ventromedian supplements bearing hiatus. The genus Aporcelaimoides is restored, its diagnosis emended, and three species of Sectonema, namely Sectonema amazonicum, Sectonema haguei and Sectonema moderatum, transferred to it. An updated list of its species, a key to their identification and a tabular compendium with the most important morphometric features are also presented.     

Effect of leucine on enzymes of the tryptophan-niacin metabolic pathway in rat liver and kidney

Dietary excess of leucine affects tryptophan–niacin metabolism adversely and has thus been implicated in the etiology of pellagra. To understand the biochemical basis of leucine-induced changes in tryptophan–niacin metabolism the effect of leucine on enzymes of tryptophan–niacin metabolism was investigated. Excess of leucine in the diet had no effect on rat liver 3-hydroxyanthranilate oxygenase and nicotinate phosphoribosyltransferase but significantly decreased the activity of quinolinate phosphoribosyltransferase of rat liver and kidney. The activities of tryptophan oxygenase in liver and picolinate carboxylase in kidney were significantly higher in leucine-fed animals than in the controls. Also, oxidation of [U-¹⁴C]tryptophan in vivo was higher in leucine-fed animals. Increased picolinate carboxylase and decreased quinolinate phosphoribosyltransferase activities would result in a decrease in NAD formation from dietary tryptophan. Lowered NAD formation from tryptophan particularly when the niacin concentrations in the diet are marginal would result in a state of conditioned niacin deficiency.     

A physiological significance of the functional interaction between Mus81 and Rad27 in homologous recombination repair

Fen1 and Mus81–Mms4 are endonucleases involved in the processing of various DNA structural intermediates, and they were shown to have genetic and functional interactions with each other. Here, we show the in vivo significance of the interactions between Mus81 and Rad27 (yeast Fen1). The N-terminal 120 amino-acid (aa) region of Mus81, although entirely dispensable for its catalytic activity, was essential for the abilities of Mus81 to bind to and be stimulated by Rad27. In the absence of SGS1, the mus81_Δ120N mutation lacking the N-terminal 120 aa region exhibited synthetic lethality, and the lethality was rescued by deletion of RAD52, a key homologous recombination mediator. These findings, together with the fact that Sgs1 constitutes a redundant pathway with Mus81–Mms4, indicate that the N-terminus-mediated interaction of Mus81 with Rad27 is physiologically important in resolving toxic recombination intermediates. Mutagenic analyses of the N-terminal region identified two distinct motifs, named N21–26 (aa from 21–26) and N108–114 (aa from 108–114) important for the in vitro and in vivo functions of Mus81. Our findings indicate that the N-terminal region of Mus81 acts as a landing pad to interact with Rad27 and that Mus81 and Rad27 work conjointly for efficient removal of various aberrant DNA structures.     

Differences in the Activities of Eight Enzymes from Ten Soil Fungi and Their Possible Influences on the Surface Structure,Functional Groups,and Element Composition of Soil Colloids

How soil fungi function in soil carbon and nutrient cycling is not well understood by using fungal enzymatic differences and their interactions with soil colloids. Eight extracellular enzymes, EEAs (chitinase, carboxymethyl cellulase, β-glucosidase, protease, acid phosphatase, polyphenol oxidase, laccase, and guaiacol oxidase) secreted by ten fungi were compared, and then the fungi that showed low and high enzymatic activity were co-cultured with soil colloids for the purpose of finding fungi-soil interactions. Some fungi (Gomphidius rutilus, Russula integra, Pholiota adiposa, and Geastrum mammosum) secreted 3–4 enzymes with weak activities, while others (Cyathus striatus, Suillus granulate, Phallus impudicus, Collybia dryophila, Agaricus sylvicola, and Lactarius deliciosus) could secret over 5 enzymes with high activities. The differences in these fungi contributed to the alterations of functional groups (stretching bands of O-H, N-H, C-H, C = O, COO- decreased by 11–60%, while P = O, C-O stretching, O-H bending and Si-O-Si stretching increased 9–22%), surface appearance (disappearance of adhesive organic materials), and elemental compositions (11–49% decreases in C1s) in soil colloids. Moreover, more evident changes were generally in high enzymatic fungi (C. striatus) compared with low enzymatic fungi (G. rutilus). Our findings indicate that inter-fungi differences in EEA types and activities might be responsible for physical and chemical changes in soil colloids (the most active component of soil matrix), highlighting the important roles of soil fungi in soil nutrient cycling and functional maintenance.     

DNA bending induced by DNA (cytosine-5) methyltransferases

DNA bending induced by six DNA (cytosine-5) methyltransferases was studied using circular permutation gel mobility shift assay. The following bend angles were obtained: M.BspRI (GG^m5CC), 46–50°; M.HaeIII (GG^m5CC), 40–43°; M.SinI (GGW^m5CC), 34–37°; M.Sau96I (GGN^m5CC), 52–57°; M.HpaII (C^m5CGG), 30°; and M.HhaI (G^m5CGC), 13°. M.HaeIII was also tested with fragments carrying a methylated binding site, and it was found to induce a 32° bend. A phase-sensitive gel mobility shift assay, using a set of DNA fragments with a sequence-directed bend and a single methyltransferase binding site, indicated that M.HaeIII and M.BspRI bend DNA toward the minor groove. The DNA curvature induced by M.HaeIII contrasts with the lack of DNA bend observed for a covalent M.HaeIII–DNA complex in an earlier X-ray study. Our results and data from other laboratories show a correlation between the bending properties and the recognition specificities of (cytosine-5) methyltransferases: enzymes recognizing a cytosine 3′ to the target cytosine tend to induce greater bends than enzymes with guanine in this position. We suggest that the observed differences indicate different mechanisms employed by (cytosine-5) methyltransferases to stabilize the helix after the target base has flipped out.     

Essential Features of the Class V Myosin from Budding Yeast for ASH1 mRNA Transport

Myo4p, a single-headed and nonprocessive class V myosin in budding yeast, transports >20 different mRNAs asymmetrically to the bud. Here, we determine the features of the Myo4p motor that are necessary for correct localization of ASH1 mRNA to the daughter cell, a process that also requires the adapter protein She3p and the dimeric mRNA-binding protein She2p. The rod region of Myo4p, but not the globular tail, is essential for correct localization of ASH1 mRNA, confirming that the rod contains the primary binding site for She3p. The requirement for both the rod region and She3p can be bypassed by directly coupling the mRNA-binding protein She2p to Myo4p. ASH1 mRNA was also correctly localized when one motor was bound per dimeric She2p, or when two motors were joined together by a leucine zipper. Because multiple mRNAs are cotransported to the bud, it is likely that this process involves multiple motor transport regardless of the number of motors per zip code. Our results show that the most important feature for correct localization is the retention of coupling between all the members of the complex (Myo4p–She3p–She2p–ASH1 mRNA), which is aided by She3p being a tightly bound subunit of Myo4p.     

Telomere-binding and Stn1p-interacting activities are required for the essential function of Saccharomyces cerevisiae Cdc13p

Yeast Saccharomyces cerevisiae Cdc13p is the telomere-binding protein that protects telomeres and regulates telomere length. It is documented that Cdc13p binds specifically to single-stranded TG_1–3 telomeric DNA sequences and interacts with Stn1p. To localize the region for single-stranded TG_1–3 DNA binding, Cdc13p mutants were constructed by deletion mutagenesis and assayed for their binding activity. Based on in vitro electrophoretic mobility shift assay, a 243-amino-acid fragment of Cdc13p (amino acids 451–693) was sufficient to bind single-stranded TG_1–3 with specificity similar to that of the native protein. Consistent with the in vitro observation, in vivo one-hybrid analysis also indicated that this region of Cdc13p was sufficient to localize itself to telomeres. However, the telomere-binding region of Cdc13p (amino acids 451–693) was not capable of complementing the growth defects of cdc13 mutants. Instead, a region comprising the Stn1p-interacting and telomere-binding region of Cdc13p (amino acids 252–924) complemented the growth defects of cdc13 mutants. These results suggest that binding to telomeres by Cdc13p is not sufficient to account for the cell viability, interaction with Stn1p is also required. Taken together, we have defined the telomere-binding domain of Cdc13p and showed that both binding to telomeres and Stn1p by Cdc13p are required to maintain cell growth.     

Characterization of Leucine Auxotrophs of the White Rot Basidiomycete Phanerochaete chrysosporium

Six leucine auxotrophic strains of the white rot basidiomycete Phanerochaete chrysosporium were characterized genetically and biochemically. Complementation studies involving the use of heterokaryons identified three leucine complementation groups. Since all of the leucine auxotrophs grew on minimal medium supplemented with α-ketoisocaproate as well as with leucine, the transaminase catalyzing the last step in the leucine pathway was apparently normal in all strains. Therefore, the wild-type, auxotrophic, and several heterokaryotic strains were assayed for the activities of the other enzymes specific to leucine biosynthesis. Leu2 and Leu4 strains (complementation group I) lacked only α-isopropylmalate synthase activity; Leu3 and Leu6 strains (group III) lacked isopropylmalate isomerase activity; and Leu1 and Leu5 strains (group II) lacked β-isopropylmalate dehydrogenase. Heterokaryons formed from leucine auxotrophs of different complementation groups had levels of activity for all three enzymes similar to those found in the wild-type strain.     

The kinetics of enzyme changes in yeast under conditions that cause the loss of mitochondria

1. Aerobically grown yeast having a high activity of glyoxylate-cycle, citric acid-cycle and electron-transport enzymes was transferred to a medium containing 10% glucose. After a lag phase of 30min. the yeast grew exponentially with a mean generation time of 94min. 2. The enzymes malate dehydrogenase, isocitrate lyase, succinate–cytochrome c oxidoreductase and NADH–cytochrome c oxidoreductase lost 45%, 17%, 27% and 46% of their activity respectively during the lag phase. 3. When growth commenced pyruvate kinase, pyruvate decarboxylase, alcohol dehydrogenase, glutamate dehydrogenase (NADP⁺-linked) and NADPH–cytochrome c oxidoreductase increased in activity, whereas aconitase, isocitrate dehydrogenase (NAD⁺- and NADP⁺-linked), α-oxoglutarate dehydrogenase, fumarase, malate dehydrogenase, succinate–cytochrome c oxidoreductase, NADH–cytochrome c oxidoreductase, NADH oxidase, NADPH oxidase, cytochrome c oxidase, glutamate dehydrogenase (NAD⁺-linked), glutamate–oxaloacetate transaminase, isocitrate lyase and glucose 6-phosphate dehydrogenase decreased. 4. During the early stages of growth the loss of activity of aconitase, α-oxoglutarate dehydrogenase, fumarase and glucose 6-phosphate dehydrogenase could be accounted for by dilution by cell division. The lower rate of loss of activity of isocitrate dehydrogenase (NAD⁺- and NADP⁺-linked), glutamate dehydrogenase (NAD⁺-linked), glutamate–oxaloacetate transaminase, NADPH oxidase and cytochrome c oxidase implies their continued synthesis, whereas the higher rate of loss of activity of malate dehydrogenase, isocitrate lyase, succinate–cytochrome c oxidoreductase, NADH–cytochrome c oxidoreductase and NADH oxidase means that these enzymes were actively removed. 5. The mechanisms of selective removal of enzyme activity and the control of the residual metabolic pathways are discussed.     

Carbon Sequestration Potential in Stands under the Grain for Green Program in Southwest China

The Grain for Green Program (GGP) is the largest afforestation and reforestation project in China in the early part of this century. To assess carbon sequestration in stands under the GGP in Southwest China, the carbon stocks and their annual changes in the GGP stands in the region were estimated based on the following information: (1) collected data on the annually planted area of each tree species under the GGP in Southwest China from 1999 to 2010; (2) development of empirical growth curves and corresponding carbon estimation models for each species growing in the GPP stands; and (3) parameters associated with the stands such as wood density, biomass expansion factor, carbon fraction and the change rate of soil organic carbon content. Two forest management scenarios were examined: scenario A, with no harvesting, and scenario B, with logging at the customary rotation followed by replanting. The results showed that by the years 2020, 2030, 2040, 2050 and 2060, the expected carbon storage of the GGP stands in Southwest China is 139.58 TgC, 177.50–207.55 TgC, 196.86–259.65 TgC, 240.45–290.62 TgC and 203.22–310.03 TgC (T = 10¹²), respectively. For the same years, the expected annual change in carbon stocks is 7.96 TgCyr⁻¹, −7.95–5.95 TgCyr⁻¹, −0.10–4.67 TgCyr⁻¹, 4.31–2.24 TgCyr⁻¹ and −0.02–1.75 TgCyr⁻¹, respectively. This indicates that the stands significantly contribute to forest carbon sinks in this region. In 2060, the estimated carbon stocks in the seven major species of GGP stands in Southwest China are 4.16–13.01 TgC for Pinus armandii, 6.30–15.01 TgC for Pinus massoniana, 11.51–13.44 TgC for Cryptomeria fortunei, 15.94–24.13 TgC for Cunninghamia lanceolata, 28.05 TgC for Cupressus spp., 5.32–15.63 TgC for Populus deltoides and 5.87–14.09 TgC for Eucalyptus spp. The carbon stocks in these seven species account for 36.8%–41.4% of the total carbon stocks in all GGP stands over the next 50 years.     

Nutritional Evaluation of Commercially Important Fish Species of Lakshadweep Archipelago,India

Estimation of nutrition profile of edible fishes is essential and thus a bio-monitoring study was carried out to find out the nutritional composition of commonly available fishes in Agatti Island water of Lakshadweep Sea. Protein, carbohydrate, lipid, ash, vitamin, amino acid and fatty acid composition in the muscle of ten edible fish species were studied. Proximate analysis revealed that the protein, carbohydrate, lipid and ash contents were high in Thunnus albacares (13.69%), Parupeneus bifasciatus (6.12%), Hyporhamphus dussumieri (6.97%) and T. albacares (1.65%), respectively. Major amino acids were lysine, leucine and methionine, registering 2.84–4.56%, 2.67–4.18% and 2.64–3.91%, respectively. Fatty acid compositions ranged from 31.63% to 38.97% saturated (SFA), 21.99–26.30% monounsaturated (MUFAs), 30.32–35.11% polyunsaturated acids (PUFAs) and 2.86–7.79% branched fatty acids of the total fatty acids. The ω-3 and ω-6 PUFAs were ranged 13.05–21.14% and 6.88–9.82% of the total fatty acids, respectively. Hence, the fishes of Lakshadweep Sea are highly recommended for consumption, since these fishes are highly enriched with nutrition. The results can be used as a baseline data for comparing the various nutritional profiles of fishes in future.     

Characterization of a cis-acting regulatory element in the protein coding region of thymidylate synthase mRNA

Thymidylate synthase (TS) functions as an RNA-binding protein by interacting with two different sequences on its own mRNA. One site is located in the 5′-upstream region of human TS mRNA while the second site is located within the protein coding region corresponding to nt 434–634. In this paper, a 70 nt RNA sequence, corresponding to nt 480–550, was identified that binds TS protein with an affinity similar to that of full-length TS mRNA and TS434–634 RNA. In vitro translation studies confirmed that this sequence is critical for the translational autoregulatory effects of TS. To document in vivo biological significance, TS sequences contained within this region were cloned onto the 5′-end of a luciferase reporter plasmid and transient transfection experiments were performed using H630 human colon cancer cells. In cells transfected with p644/TS434–634 or p644/TS480–550, luciferase activity was decreased 2.5-fold when compared to cells transfected with p644 plasmid alone. Luciferase mRNA levels were identical for each of these conditions as determined by RNase protection and RT–PCR analysis. Immunoprecipitation of TS ribonucleoprotein complexes revealed a direct interaction between TS protein and TS480–550 RNA in transfected H630 cells. Treatment with 5-fluorouridine resulted in a nearly 2-fold increase in luciferase activity only in cells transfected with p644/TS434–634 and p644/TS480–550. This study identifies a 70 nt TS response element in the protein coding region of TS mRNA with in vitro and in vivo translational regulatory activity.     

Dependence on Mating Type for the Overproduction of Iso-2-Cytochrome c in the Yeast Mutant CYC7–H2

The CYC7–H2 mutation causes an approximately 20-fold overproduction of iso–2–cytochromo c in a and α haploid strains of the yeast Saccharomyces cerevisiae due to an alteration in the nontranslated regulatory region that is presumably contiguous with the structural region. In this investigation, we demonstrated that heterozygosity at the mating type locus, a/α or a/a/α/α, prevents expression of the overproduction, while homozygosity, a/a and α/α, and hemizygosity, a/0 and α/0, allow full expression of the CYC7–H2 mutation, equivalent to the expression observed in a and α haploid strains. There is no decrease in the overproduction of iso-2-cytochrome c in a/α diploid strains containing either of the other two similar mutations, CYC7–H1 and CYC7–H3. It appears as if active expression of one or another of the mating-type alleles is required for the overproduction of iso-2-cytochrome c in CYC7–H2 mutants.     

Family 46 Carbohydrate-binding Modules Contribute to the Enzymatic Hydrolysis of Xyloglucan and β-1,3–1,4-Glucans through Distinct Mechanisms

Structural carbohydrates comprise an extraordinary source of energy that remains poorly utilized by the biofuel sector as enzymes have restricted access to their substrates within the intricacy of plant cell walls. Carbohydrate active enzymes (CAZYmes) that target recalcitrant polysaccharides are modular enzymes containing noncatalytic carbohydrate-binding modules (CBMs) that direct enzymes to their cognate substrate, thus potentiating catalysis. In general, CBMs are functionally and structurally autonomous from their associated catalytic domains from which they are separated through flexible linker sequences. Here, we show that a C-terminal CBM46 derived from BhCel5B, a Bacillus halodurans endoglucanase, does not interact with β-glucans independently but, uniquely, acts cooperatively with the catalytic domain of the enzyme in substrate recognition. The structure of BhCBM46 revealed a β-sandwich fold that abuts onto the region of the substrate binding cleft upstream of the active site. BhCBM46 as a discrete entity is unable to bind to β-glucans. Removal of BhCBM46 from BhCel5B, however, abrogates binding to β-1,3–1,4-glucans while substantially decreasing the affinity for decorated β-1,4-glucan homopolymers such as xyloglucan. The CBM46 was shown to contribute to xyloglucan hydrolysis only in the context of intact plant cell walls, but it potentiates enzymatic activity against purified β-1,3–1,4-glucans in solution or within the cell wall. This report reveals the mechanism by which a CBM can promote enzyme activity through direct interaction with the substrate or by targeting regions of the plant cell wall where the target glucan is abundant.